1. Field of the Invention
The invention concerns a method and an apparatus for illuminating points on a medium, said method and said apparatus being of the type defined in the characterizing portion of claim 1, respectively. In a preferred embodiment the apparatus of the invention is used for selective illumination of points on the surface of a medium, and said apparatus may comprise a central control unit having a store in which the information on a desired illumination pattern is stored.
2. Description of the Prior Art
It is known to expose photosensitive media with light to transfer information to it. This was originally done by illuminating the medium through a partly transparent original containing the desired information. To rationalize this process techniques have subsequently been developed, where a laser beam scans the medium for exposure, while the laser beam is modulated for the medium to obtain the correct illumination in all points. The information on the illumination pattern of the medium is now contained in a central control unit and is used for modulating the laser. For a satisfactory image quality to be obtained on the medium, the mechanical system retaining the original and the light source with respect to each other during exposure must satisfy strict stability requirements. For the required accuracy to be obtained, the medium for exposure is often arranged in a device of the type usually called "Internal Laser Image Drum Plotter". Devices of this type are described i.a. in U.S. Pat. Nos. 4,852,709, 4,595,957 and 3,958,250. Devices of this type provide a satisfactory image quality, but they have a very complex mechanical structure, because the light source and the medium are to be moved very accurately with respect to each other. Consequently, devices of this type have a relatively high cost price, which puts a limit to the number of users.